The present invention is directed to a method for manufacturing a low-impedance, planar metallization for LSI semiconductor circuits. More particularly, the present invention is directed to a method for manufacturing a low-impedance, planar metallization for LSI semiconductor circuits composed of aluminum or of an aluminum alloy, wherein aluminum is used as a via hole filler and a double layer composed of titanium/titanium nitride is used as a diffusion barrier layer. The present invention also is directed to a low-impedance metallization manufactured pursuant to this method.
As a result of the higher integration of semiconductor circuits (transistors, capacitors) on a chip, the available space for individual elements is constantly decreasing. A multi-layer wiring is therefore also increasingly necessary in MOS circuits in the wiring of VLSI semiconductor circuits. A prerequisite therefor is a planarization of the surface of the individual layer components (insulating oxides, metallization layers).
In order to determine the reliability of the metallization, a number of factors must be considered. The reliability of metallization is determined by the electromigration resistance of the material, the hillock growth (which can lead to short circuits), by mechanical stresses in the layer, by void formation, and by corrosion, as well as, by the edge coverage over the steps and in the via hole.
The present invention provides a method for manufacturing a low-impedance, planarizing wiring with aluminum for filling via holes (i.e., contact holes in insulating layers between two metallization levels) and a sandwich aluminum structure as an interconnect level that meets the requirements of high reliability and can be manufactured with a conventional structuring method.
Aluminum alloy layers such as aluminum/silicon alloys, aluminum/silicon/titanium alloys, aluminum/copper, and aluminum/silicon/copper alloys have been heretofore used for the metallization of integrated circuits. For example, European Patent Application No. 0 110 401 discloses such an aluminum alloy based on an aluminum/silicon/titanium alloy having a part of one percent through two percent by weight of silicon and a titanium additive of less than 0.5% by weight.
European Patent Application No. 0 199 030 discloses a via hole filling method for the manufacture of reliable aluminum contacts, loadable with high current densities, wherein aluminum is used that is deposited surface-wide by low-pressure vapor phase deposition and is etched back again to the level of the via hole. No decrease in the layer thickness in the via hole occurs because of the aluminum fill situated in the via hole, so that a current load is likewise not critical at these locations. There is, however, the risk of aluminum spiking that can lead to substrate shorts.
To improve the planarization and the reliability of low-impedance interconnects of aluminum, German Patent Application No. P36 40 656.2 proposes the use of tungsten as a via hole filler and the use of a metallization pattern as an interconnect layer. The metallization pattern contains a molybden silicide layer and a titanium/titanium nitride double layer as an underlayer, because of the high specific resistance of tungsten and because of the poor bondability and structurability.
Given a high aspect ratio, and unfavorable via hole shape, a polyimide layer must frequently be deposited due to the formation of what are referred to as key holes. The polyimide layer must be etched back again in order to avoid an over-etching of the tungsten in the via hole. Moreover, there is a further disadvantage in that the metallic contact between the aluminum and tungsten is undefined. Furthermore, the tungsten can potentially contain fluorine, due to tungsten hexafluoride, that can lead to aluminum corrosion.
Further, the methods disclosed in European Patent Application No. 0 199 030 and German Patent Application No. P36 40 656.2 suffer a disadvantage due to their starting materials. Both methods, because of the initial materials employed, such as tungsten hexafluoride and triisobutyl aluminum, provide an increased risk during the manufacturing process; due to the high toxicity of tungsten hexafluoride and ignitability of triisobutyl aluminum.